1. Field of the Invention
This invention relates to gas sensors used in leak detectors or alarm units for detecting a fuel gas such as town gas or liquefied petroleum gas.
2. Description of the Related Art
Known semiconductor type gas sensors used in alarm units for detecting leakage of a fuel gas have sensitivity also to other gases than the fuel gas to be detected, such as hydrogen and alcohol gases. This is a major cause of false alarms. Thus, there are demands for semiconductor type gas sensors having reduced sensitivity to such interfering gases as hydrogen gas and alcohol gas, and excellent selectivity for a fuel gas or the like.
In order to obtain a semiconductor type gas sensor having selective sensitivity to a particular gas, one of various active substances is included in a surface layer of the sensor to act as a filter layer or a filter layer of an active substance is additionally formed on the sensor. FIG. 10 shows an example of known semiconductor type gas sensors 5' having a filter layer 3. This sensor comprises a metal electrode 4 formed of platinum, which is covered by a metallic oxide semiconductor layer 1' of tin oxide (SnO.sub.2) , zinc oxide (ZnO) or the like. The metallic oxide semiconductor layer 1' is covered by a filter layer 3 including a support formed of alumina, for example, and supporting an active precious metal such as platinum or palladium.
With such a semiconductor type gas sensor 5', attention has been paid only to the metallic oxide semiconductor layer 1' or to its outer surface. In order to vary the sensor's sensitivity with regard to gas sensitivity, therefore, it has been conventional practice to mix an additive into the entire metallic oxide semiconductor layer 1' or apply it selectively to its outer surface. Alternatively, a plurality of such semiconductor layers formed on the sensor or a filter layer is added to the sensor.
Attempts have been made heretofore to improve the gas selectivity of the semiconductor type gas sensor with efforts made only in connection with the interface reaction between the gas atmosphere and metallic oxide semiconductor layer 1'. That is, although the semiconductor type gas sensor includes an interface M between the metallic oxide semiconductor layer 1' and the metal electrode 4, this interface M has been considered to have no direct relevance to gas sensitivity characteristics because the gas to be detected reacts within the semiconductor layer 1' before reaching the interface M.
The known semiconductor type gas sensor having a filter layer as noted above has the more complicated construction and requires the greater number of manufacturing steps for the filter layer, and therefore is not well suited for mass production. Further, such a semiconductor type gas sensor has a structural problem that combustion takes place at the filter layer upon contact with a high concentration fuel gas, whereby the semiconductor inside deteriorates through exposure to the resulting heat of reaction.